1. Field of Invention
The present invention relates to an automatic exposure control method and an automatic exposure compensation apparatus, and particularly to an automatic exposure control method and an automatic exposure compensation apparatus used for setting the exposure compensation value of an image.
2. Description of the Related Art
The components of a color image processing system mainly include an image acquisition unit, an image regression and processing unit, an image display unit, and an image compression unit. In which, the image acquisition unit is formed by an image sensor, which is a two-dimensional array device formed by a plurality of photo-diodes and which converts a detected optical intensity into an electronic signal to be provided for the subsequent image regression and processing unit to perform the appropriate image processing. Since the direction and intensity of a light source for a photographic scene is varied, an excessively bright or dark principal object frame is as a result. An exposure control is intended to control the intensity and time duration of the supplementary lighting under various scene conditions for correcting the excessively bright or dark principal object frame caused by the luminance difference between the principal object and the background.
Accordingly, many manufactures have developed a number of light-metering methods to improve the exposure of the principal object in a photographic frame. However, a general-purpose light-metering method is not capable of effectively alleviating the problem. As a prior art, for example, Matsushita Electric Industrial Co. provided the U.S. Pat. No. 5,703,644 with title of “Automatic Exposure Control Apparatus”, in which a frame is divided into a plurality of smaller element blocks having a calculated average luminance of each block. Afterwards, the calculated average luminance of the blocks are sequentially arranged by the corresponding magnitude; and a histogram of the block count versus the average luminance is generated, followed by the classification of all the element blocks into a principal object region and a background region. Then, based on the calculated average luminance of the selected smaller element blocks, the exposure correction of the backlighting and the strong front lighting scene is performed. Using the above scheme, an up-down and left-right positional or orientational variations would likely produce a misjudgement of image frame of a scene. FIG. 1 is, for example, a diagram showing a misjudgement of an image frame of a scene using the method provided by U.S. Pat. No. 5,703,644. According to the aforementioned patent, a scene 102 is divided into two regions, the principal object region 104 locating in the center portion of the scene and the background region 106 locating in the peripheral portion of the scene. When the subject (the principal object) to be photographed is moved to the left from a position 108 to a position 110, the current principal object region 104 would be substituted by a background. Since the background is a brighter region (for example, the background is the sky), the sky would be misjudged to be the principal object to have excessive luminance and an incorrect negative compensation, i.e. a deducted lighting, would be performed, which leads to the above-described misjudgement of an image frame of a scene.
To overcome the disadvantage of an excessively bright or dark principal object in a photographic frame, another prior art by Sony Corporation provided the U.S. Pat. No. 6,690,424 in title of “Apparatus for Controlling the Exposure of an Image Pickup Plane in a Camera.” Indeed, the art provided by the aforementioned patent is for an exposure control method for backlighting scenes, which sets two luminance reference values according to the average luminance value of the entire frame and divides the frame into a principal object region and a background region based on the two luminance reference values, followed by calculating an average luminance difference between the entire frame and the principal object region. By using the aforementioned difference value, an exposure compensation gain can be determined. The aforementioned method is able to improve the accuracy of judgement of image frame in a scene when the principal object has an up-down and left-right positional or orientational variation. However, the aforementioned method would still fail to provide a correct judgment when a principal object is under strong front lighting. FIG. 2 is, for example, a diagram showing a misjudgement of an image frame of a scene using U.S. Pat. No. 6,690,424. In which, the abscissa represents the average luminance (Y), and the ordinate represents the pixel count. The two lines with down arrows indicate the two luminance reference values, 202 and 204; and Y indicates a luminance difference between the two luminance reference values. For a distribution curve 206 in the diagram, the method provided by the aforementioned patent would provide the judgement of the photographic scene under a backlighting situation and a positive compensation is performed accordingly. However, a similar distribution curve 206 under strong front-lighting can be presented as well; therefore, a misjudgment of the photographic scene as a back-lighting situation and a positive compensation given based on the method of the aforementioned patent would make the result even worse, so that an originally strong front lighting scene becomes even more brighter, thus is indicative of a case of misjudgement of a image frame of a scene.
Furthermore, another U.S. Pat. No. 6,853,806 provided by Olympus Optical Co. having title of “Camera with an Exposure Control Function” is another conventional exposure control method for back-lighting scenes. In which, a distance metering sensor is used; and the average luminance of an entire frame is calculated for determining the position of a principal object in the frame, and followed by using a back-lighting assessment unit for determining the intensity and duration of an exposure. To carry out the method in the aforementioned patent, a camera must be equipped with a distance metering sensor, which results in structural complexity and increased cost.